1. Field of the Invention
The present invention relates to a helical compression spring for a vehicle suspension, and more particularly to a mounting system for the helical compression spring that maintains the spring in a linear orientation over the entire range of compression.
2. Description of the Related Arts
In general, a vehicle suspension is provided with a substantially helical compression spring, where the compression spring works in concert with a shock absorber. The compression spring can be substantially coaxial to a telescoping strut, or a shock absorber that simultaneously serves as a telescoping strut. In the case where the compression spring is mounted having an annular shock absorber (i.e., a coil-over or MacPherson strut), the spring is typically held between a lower spring seat that is affixed to the piston, and an upper seat that is usually affixed to the frame. In this configuration, the shock absorber has some degree of movement within the diameter to the coils of the spring to adjust to movement of the suspension. Alternatively, the compression spring can be used independently with a shock absorber, where the shock absorber and the spring are not combined in a unified part. The obvious advantages of a shock absorber and a compression spring working independently are that there are no interdependent geometrical size limitations. The shape and size of the spring need not be determined by the shape and size of the shock absorber. The advantages of combining the dampening shock absorber and the spring are that they tend to take up less space, and the combination would appear to be more effective because the reactive forces are unified. Typically, a suspension system for a two axle vehicle has one shock absorber and one spring per wheel. This is true, whether the suspensions are independent or unified, as in the case of a solid axle. All suspensions share the common feature that when the wheel is positively stressed by a change in the static loaded position by impact with a raised surface, the wheel moves upward to a position where the stress force is about equal to the resistive force produced by the spring and shock absorber. With modern suspensions, the movement of the wheel is substantially perpendicular to the road so that the traction remains constant and there is not a big difference in wear characteristics when comparing one side of the tire to the other. The control arm system, which is an articulating strut or a combination of articulating struts, limits the range of motion of the wheel (and axle where appropriate) to substantially perpendicular movement. The movement of the articulating strut is substantially radial, moving through an arc as the wheel (and the axle in some cases) moves upward or downward. The articulating strut incurs an incremental bending moment as a result of the movement of the wheel. The movement of the wheel produces a stress force at a stress point (e.g., incremental from the static load) that is countered by a resistance force at a resistance point (e.g., incremental from the static load), where the resistance point is offset from the stress point by a distance X. The stress point is typically designated at the center of the tread width of the tire. The resistance force is substantially generated by the spring, and so the resistance point is the location where the spring, or an extending strut of the spring, is in connection with the articulating strut. In the case where the compression spring is mounted on the shock absorber, then the resistance point is where the shock absorber attaches to the articulating strut. The shock absorber reacts to the bending moments, which in turn causes the spring to react. As the motion of the articulating strut is angular, the shock absorber is usually pivotally connected to facilitate linear movement of the rod through the piston. The piston is typically pivotally connected to the articulating strut, and the rod is pivotally connected to the chassis or frame. This type of suspension is described by Halford et al., U.S. Pat. No. 2,992,015, and is commonly known as a coil-over strut, or MacPherson strut. Muhr et al., in U.S. Pat. No. 4,903,985, in the background of the disclosure examines the bending moment of a shock absorber-spring combination. Muhr et al., reports that others have taught that the further the resistance point is from the stress point, the greater the bending moment. By way of example, if the shock absorber is connected to the control arm X inches from the inside rim of a Y inch wide tire, and the spring is coaxial with the strut, then the point of resistance is X inches, plus half the width of the tire (Y/2) for a total distance of inches (X+Y/2) from the point of stress. If the tire is wider, the difference is larger. The greater the difference in distance between the point of stress and the point of resistance, then the greater the strain, and the greater the bending moment of the strut. In terms of a shock absorber, the higher the bending moment, the greater the drag resistance necessary to move the rod through the piston. To correct for the imbalance, a solution has been to angle the spring so that the centerline of the spring is aligned with the point of stress. Accordingly, using this type of suspension, wherein the spring is positioned between a lower spring seat that is attached to the piston and an upper spring seat that is mounted to the frame, it has been taught to arrange the cylindrical helical compression spring to be offset from the shock absorber axis, such that the centerline of the spring is preferably aligned with the stress point. Vectorially aligning the spring reputedly reduces the bending moment acting upon the shock absorber. In the static loaded state, the piston rod of the shock absorber is reported to be largely free of shear forces, however, the configuration does not ameliorate the problem during the non-static state. When the wheel moves either upward or downward, the spring alignment is lost because the alignment of the resistance point does not adjust for the position change of the wheel. When the wheel flexes upward, the control arm of the suspension pivots upward through an arc. For instance, a one foot control arm deflected 25° will travel through a 5 inch secant. If the angle of a coil-over strut is approaching 60° with respect to the control arm, then a strut that is approximately 1.5 times the length of the control arm will deflect to a more acute angle of approximately 50°. The overall length of the strut will compress about 22%. A spring having a length that is two-thirds the length of the strut will be compressed 33%. A deflection of this magnitude creates a large bending moment on the strut, as it is being flexed through a 25° arc in a very short period of time. Angling the spring possibly helps under static conditions, but because the stress point changes as the control arm pivots, the spring is often out of alignment to produce the most effective resistance, and the level of performance is not as good as it would be if the spring were properly aligned. While the coil-over strut has the apparent advantage that the spring is substantially coaxial with the helical compression spring, it is limited in that the shock absorber must be sufficiently robust to serve as both a pneumatic cylinder dampener and as the strut that transfers the suspension forces between the control arm and the chassis. As previously discussed, the bending moment produced during a deflection of the wheel can cause frictional resistance, which in turn wears out the shock, creates noise, and is a limiting factor as to the force it takes to bottom out the suspension.
In the second type of suspension, wherein the helical compression spring and the shock absorber are independently connected to the articulating member, as before it is desired that the stress forces and the resistance forces be aligned. In the case of independent suspensions, the articulating member is either an axle arm, a control arm, or both. In all cases, the articulating strut traverses through an arc as the wheel deflects either upward or downward to smooth out the road surface variations. What is needed is an apparatus that will provide a spring that will compress or extend linearly, even as the articulating strut moves angularly. A linearly deflected spring reacts along its centerline, thereby producing a substantially linear response to the stressing forces. When the spring is linearly compressed, its coils act uniformly in response to the stressing force, and the spring is not curved, and the coils are not unequally compressed. Since the spring is not curved there is no transverse reaction generated when the spring recoils. The attendant shock absorber provides dampening to vibrations that are aligned with the piston, and is largely ineffective at dampening transverse vibrations, so that when the spring deflects from linearity the performance of the shock absorber deteriorates. Furthermore, since the compression and recoil are uniform, the spring will have a longer wear life.
In a variation of the second type of suspension, the helical compression springs and the shock absorbers are independently connected to a solid axle. This type of suspension is commonly associated with heavy load vehicles like trucks, but it is also used with high performance vehicles, and particularly muscle cars or dragsters where there is a lot of torque. Ford's 2005 Mustang has a solid rear axle three-link suspension. The solid rear axle suspension is robust, maintains constant track, toe-in and camber relative to the road surface, and it keeps body roll well under control. A central torque control arm is fastened to the upper front end of the differential, while trailing arms are located near each end of the axle. The lightweight, tubular Panhard rod is parallel to the axle and attached at one end to the body, and at the other end to the axle. The Panhard rod stabilizes the rear axle side-to-side, as the wheels move through jounce and rebound. It also firmly controls the axle during hard cornering. The shocks are located on the outside of the rear structural rails, near the wheels, reducing the lever effect of the axle and allowing more precise, slightly softer tuning of the shock valves. As configured, the compression springs are connected directly to the axle, inboard of the shock absorbers. The performance characteristics of a rigid axle are such that often when one spring is in extension, the spring on the opposing side is in compression. The Panhard rod tends to compensate for this by partially compressing the spring in extension, however, it does not eliminate this characteristic entirely, and the axle and any longitudinal struts will be articulating through a small rotation. The compression springs will be transversely deflected. What is desired is a linear suspension spring that will compress or extend linearly, even as the articulating strut moves angularly.